Optical microphone for implantable hearing instrument

ABSTRACT

An implanted microphone is provided that allows for isolating an acoustic response of the microphone from vibration induced acceleration responses of the microphone. The present invention measures the relative motion between a microphone diaphragm, which is responsive to pressure variations in overlying media caused by acoustic forces and acceleration forces, and a cancellation element that is compliantly mounted within a housing of the microphone, which moves primarily in response to acceleration forces. When the microphone and cancellation element move substantially in unison to acceleration forces, relative movement between these elements corresponds to the acoustic response of the microphone diaphragm. This relative movement may be directly measured using various optical measuring systems.

CROSS REFERENCE

This application claims the benefit of the filing date of U.S.Provisional Application No. 61/510,201 entitled “Optical Microphone forImplantable Hearing Instrument” having a filing date of Jul. 21, 2011,the entire contents of which is incorporated herein by reference.

FIELD

The present disclosure relates to implanted microphone assemblies, e.g.as employed in hearing aid instruments, and more particularly, toimplanted microphone assemblies having improved acoustic sensitivity andreduced sensitivity to non-acoustic vibration.

BACKGROUND OF THE INVENTION

Until recently, a large number of people affected by sensorineuralhearing loss of 55 dB or more have been unable to receive adequatetherapeutic benefit from any available technology. This problem has beenalleviated to some extent by the development of a class of hearing aidsgenerally referred to as implantable hearing instruments, which include,for example, middle ear implants and cochlear implants. Generally, suchimplantable hearing instruments utilize an implanted transducer tostimulate a component of the patient's auditory system (e.g., tympanicmembrane, ossicles and/or cochlea). By way of example, one type ofimplantable transducer includes an electromechanical transducer having amagnetic coil that drives a vibratory actuator. The actuator ispositioned to interface with and stimulate the ossicular chain of thepatient via physical engagement. (See e.g. U.S. Pat. No. 5,702,342). Inthis regard, one or more bones of the ossicular chain are made tomechanically vibrate causing stimulation of the cochlea through itsnatural input, the so-called oval window.

Amongst users of implantable hearing instruments, there is a strongdesire for a small, fully implantable system. In such hearinginstruments, the entirety of the instruments of various hearingaugmentation components, including a microphone assembly, is positionedsubcutaneously on or within a patient's skull, typically at locationsproximate the mastoid process.

As may be appreciated, implantable hearing instruments that make use ofan implanted microphone require that the microphone be positioned at alocation that facilitates the receipt of acoustic signals. For suchpurposes, such implantable microphones may be typically positioned in asurgical procedure between a patient's skull and skin, at a locationrearward and upward of a patient's ear (e.g., in the mastoid region).Accordingly, the hearing instrument must overcome the difficulty ofdetecting external sounds (i.e., acoustic vibrations) after attenuationby a layer of skin. In this regard, a subcutaneously located microphonemust provide adequate acoustic sensitivity while being covered by alayer of skin between about 3 mm and 12 mm thick.

Further, a subcutaneously located microphone must also be able todiscriminate between acoustic sounds and unwanted vibrations. That is,acceleration within patient tissue (e.g., caused by tissue-bornevibration) may cause pressure fluctuations that are commingled withpressure fluctuations caused by acoustic sounds impinging on tissueoverlying an implanted microphone. This undesirable commingling ofambient acoustic signals and tissue-borne acceleration signals is at theroot of several problems facing the designers of implantable hearingsystems.

One particular problem relates to vibrations caused by the implantwearer's voice, chewing or vibration caused by the hearing instrumentitself (e.g., an electro-mechanical transducer) that may generatedistortion of a wearer's own voice, provide feedback and/or reduceacoustic sensitivity. For example, sound emanating from the vocal chordsof a person wearing an implantable hearing instrument passes through thebony structure of the head (i.e., as a vibration) and reaches theimplanted microphone of the implantable middle ear hearing system orfully implantable cochlear implant. The vibration reaches the microphoneand may induce pressure fluctuations within the skin due toacceleration. Accordingly, such pressure fluctuations may be amplifiedjust as a pressure fluctuation caused by the deflection of the skin'ssurface by an acoustic sound. In this regard, the implanted microphonedetects the combination of these two sources as a single varyingpressure. Further, in systems employing a middle ear stimulationtransducer, the microphone may produce feedback by picking up andamplifying vibration caused by the stimulation transducer. As such, thebone-borne vibration undesirably limits the maximum achievable gain ofthe implantable hearing instrument.

In order to achieve a nearly natural quality of the implant wearer'svoice and detect acoustic signals with sufficient sensitivity, animplanted microphone needs to isolate acoustic responses fromnon-acoustic vibration responses. The aim of the present invention is todesign an implantable microphone that achieves these goals.

SUMMARY OF THE INVENTION

Although all microphones possess some degree of accelerationsensitivity, unwanted responses from acceleration is not significantlylimiting to the performance of acoustic microphones, that is,microphones designed to operate in air. The inventor of the presentinvention has recognized that the same is not true, however, forsubdermal/implanted microphones as acceleration within tissue arisingfrom tissue-borne vibration (e.g., from talking or chewing) causespressure fluctuations that are combined/commingled by the implantedmicrophone with pressure fluctuations caused by external/ambient sounds.In this regard, pressure fluctuations in tissue (e.g., overlying animplanted microphone) may arise from external pressures such as ambientacoustic signals (i.e., sound) impinging on the skin as well as fromacceleration within the tissue caused by vibration. Accordingly, amethod and system for distinguishing or isolating an acoustic signalcomponent from a commingled signal is desirable.

The present inventor has recognized that previous attempts to isolate anacoustic signal from a combined acoustic and acceleration outputresponse of an implanted microphone have typically focused on separatelymeasuring acceleration forces applied to an implanted microphone andsubsequently removing a signal indicative of those acceleration forcesfrom a signal indicative of the combined output response. The inventorhas further recognized that, rather than measuring a combined acousticand acceleration output response of an implanted microphone, it ispossible to isolate and more directly measure the acoustic response.That is, rather than monitoring a combined output response, the presentinvention allows for measuring an acoustic response of an implantedmicrophone even in the presence of acceleration forces acting on themicrophone. Such measurement may significantly reduce or eliminate theneed for subsequent processing to remove undesired signals from themicrophone response. Accordingly, improved gain and hearing augmentationmay be achieved.

The inventor has recognized that microphone acoustic response isolationmay be achieved by monitoring the relative movement of the microphonediaphragm, which is subject to acoustic and acceleration forces,relative to a compliant cancellation element that moves in response toacceleration forces but has little or no response to acoustic forces.That is, when the microphone diaphragm and cancellation element havesubstantially common movement in relation to commonly appliedacceleration forces, the relative movement between these elements isprimarily due to the difference in the total forces applied to theseelements. Specifically, this difference in forces is primarily due tothe acoustic forces applied to the microphone diaphragm. Accordingly, bymonitoring relative movement between the diaphragm and the cancellationelement it is possible to isolate the acoustic response of themicrophone diaphragm from a combined response of the microphonediaphragm including response to acoustic and acceleration forces.

The inventor has further determined that such relative movement may beeffectively monitored or measured optically. That is, relative movementmay be monitored by optically measuring a path length between an insidesurface of an implantable microphone housing (e.g., inside surface ofthe diaphragm) and a cancellation element that is operative to move inrelation to acceleration forces. Changes to the path length maysubstantially correlate to the acoustic forces applied to the microphonediaphragm. In order to measure the optical path length between theinside surface of the microphone housing and the cancellation element, alight source is provided that is operative to project a beam betweenthese two elements where this beam is subsequently received by anoptical detector. This optical detector generates an output signal thatis indicative of the relative movement between these elements.Accordingly, such an output may be used to generate a stimulation ordrive signal for actuating an actuator or other stimulation element ofan implantable hearing instrument such as, without limitation, a middleear transducer, a cochlear electrode and/or a bone anchored vibratingelement. In one arrangement, generating a stimulation signal may includeadditional processing of the output to provide, for example, frequencyshaping, amplification, compression, noise reduction/cancellation orother signal conditioning, including conditioning based onpatient-specific fitting parameters.

According to a first aspect, an implantable microphone is provided. Themicrophone includes a housing that has an internal chamber and aflexible housing portion that is operative to move in response topressure variations present in media overlying the flexible housingportion. Such pressure variations may include acoustic forces,acceleration forces (e.g., tissue-borne vibrations) and/or a combinationof these forces. The microphone also includes a cancellation elementthat is disposed within the internal chamber of the housing. At least aportion of this cancellation element is operative to move relative tothe housing in response to acceleration forces that act on the housing.A light source disposed within the housing generates an output beam. Atleast a portion of the output beam is received by an optical detectorafter this portion of the output beam has travelled an optical pathbetween the light source and optical detector that includes at least onereflection off of the flexible housing portion and at least onereflection off of the cancellation element. The optical detectorgenerates an output signal that is indicative of the received light.

It will be appreciated that the movement of the flexible housingportion, which in various arrangements may be formed of a microphonediaphragm extending over an aperture in the housing, relative to thecancellation element changes the path length between these elements andthus alters the light beam as received by the detector. For instance, inone arrangement, the change in the light path modulates the light beamsuch that when an output of the optical detector is demodulated (e.g.,using an un-modulated beam) the demodulated signal generates an outputindicative of the movement of the flexible housing portion relative tothe cancellation element. In another arrangement, the change in the pathlength may alter the size (e.g., diameter or other cross-dimension) ofthe output beam as received by the optical detector. In this latterregard, an increase or decrease of the path length between the sourceand optical detector may change the cross-dimension of, for example, aconverging light beam as received at the optical detector. Accordingly,such a change in the diameter or cross dimension alters the output ofthe optical detector and is indicative of the relative movement of thediaphragm relative to the cancellation element. In any arrangement, therelative movement is indicative of the acoustic response of the flexiblehousing portion with reduced response to acceleration forces.

The output of the optical detector may be utilized to generate a drivesignal for receipt by an auditory stimulation device. In this regard,the output signal may be processed by a processor that is located withinthe microphone housing. Alternatively, the output signal may be providedto a separate housing that includes processing equipment. In eitherarrangement, additional processing may be performed to provide, forexample, frequency shaping, amplification, compression, noise reductionor other signal conditioning, including conditioning based onpatient-specific fitting parameters.

In one arrangement, the light source is formed of a laser such as alaser diode or converging laser. In a particular embodiment, the lasersource forms part of a Laser Doppler Velocimeter (LDV). In thisarrangement, an output of the laser is separated into two or moreidentical beams. One of these beams, a reference beam is provided to theoptical detector free of reflection off of the flexible housing portionand/or the cancellation element. In contrast, the other beam, a testbeam, reflects off of one or both of the vibrating elements in order tomodulate the beam. The LDV may include one or more beam splitters forsplitting and/or combining beams as well as one or more frequencyshifting elements such as, and without limitation, a Bragg cell.

In another arrangement, a convergent/converging laser is utilized toidentify path length changes between the flexible housing portion andthe cancellation element. At rest (e.g., during static conditions), thelaser generates a footprint on the photo diode having a fixedcross-dimension. During movement, this cross-dimension changes andgenerates an output signal that is indicative of the relative movementbetween the elements.

In all cases, it may be desirable that the magnitude of the response ofthe cancellation element to acceleration be chosen to substantiallymatch the response of the flexible housing portion to acceleration.Likewise, it may be desirable that the phases of these elementssubstantially match to achieve enhanced cancellation. Stated otherwise,it may be preferred that such magnitude and phase matching occur in afrequency range of interest (e.g., frequencies important to hearing).This may require that the resonant frequency of each the flexiblehousing portion and cancellation element be less than about 2000 Hz andmore preferably less than about 200 Hz. These resonant frequencies aretypically below an acoustic hearing frequency range. Further, it may bedesirable that the flexible housing portion and cancellation elementhave substantially equal resonant frequencies and/or equal dampingfactors.

The cancellation element may be any compliant structure that isoperative to move in response to acceleration forces applied to themicrophone. For instance, the cancellation element may include acompliantly supported mass (e.g., a proof or seismic mass). Inertialmovement of the proof mass in response to acceleration forces mayphysically match movement of the flexible housing portion in response toacceleration. Any appropriate means may be utilized to compliantly mountand/or damp the movement of the cancellation element including, withoutlimitation, mechanical springs, fluid bladders/cushions and/or magnets.

According to another aspect, a method is provided for isolating anacoustic response from a combined response of an implanted microphone.The method includes reflecting a first light beam off an inside surfaceof a housing of the implantable hearing instrument and/or reflectingthat light beam off a cancellation element within an internal chamber ofthe housing. The method further includes receiving the first light beamat an optical detector after a reflection off of one or both of theinside surface of the housing and cancellation element and generating anoutput signal indicative of relative movement between these elements.

The method may further include processing the output signal to generatea vibration signature of the inside surface of the housing. Thisvibration signature may be indicative of a movement of the insidesurface of the housing relative to the cancellation element caused bythe acoustic forces applied to that surface. The method may furtherinclude generating a stimulation signal utilizing the vibrationsignature where the stimulation signal is subsequently operative for usewith an actuator/stimulator of an implantable hearing instrument.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a fully implantable hearing instrument.

FIG. 2 illustrates the hearing instrument of FIG. 1 as implanted.

FIG. 3 illustrates the implantable hearing instrument of FIG. 1 aspositioned on a wearer.

FIG. 4 illustrates the combination of acoustic-induced vibration andacceleration-induced vibration by a microphone diaphragm.

FIG. 5 illustrates a mathematical depiction of the response of amicrophone and cancellation element to applied acoustic and accelerationforces.

FIG. 6A illustrates a cross-sectional side view of one embodiment of amicrophone that allows optically isolating an acoustic response of themicrophone diaphragm.

FIG. 6B illustrates a top view of the microphone of FIG. 6A.

FIG. 7 illustrates a cross-sectional side view of another embodiment ofa microphone that allows optically isolating an acoustic response of themicrophone diaphragm.

FIG. 8 illustrates an operating principle of the microphone of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made to the accompanying drawings, which at leastassist in illustrating the various pertinent features of the presentedinventions. In this regard, the following description of a hearinginstrument is presented for purposes of illustration and description.Furthermore, the description is not intended to limit the inventions tothe forms disclosed herein. Consequently, variations and modificationscommensurate with the following teachings, and skill and knowledge ofthe relevant art, are within the scope of the presented inventions. Theembodiments described herein are further intended to explain the bestmodes known of practicing the inventions and to enable others skilled inthe art to utilize the inventions in such, or other embodiments and withvarious modifications required by the particular application(s) oruse(s) of the presented inventions.

Hearing Instrument System:

FIGS. 1-3 illustrate one embodiment of a system in which various aspectsof the disclosed inventions may be incorporated. The illustratedembodiment is a fully implantable hearing instrument system thatprovides mechanical stimulation to the middle ear of a patient. As willbe appreciated, certain aspects of the presented inventions may beemployed in conjunction with different implantable systems including butnot limited to semi-implantable hearing instruments, bone conductionhearing instruments, as well as inner ear hearing instruments (e.g.,cochlear implants), and therefore, the illustrated application is forpurposes of illustration and not by way of limitation.

In the illustrated system, a biocompatible implant housing 100 islocated subcutaneously on a patient's skull. The implant housing 100includes an implanted signal receiver 118 (e.g., comprising a coilelement) and is interconnected to a microphone assembly 130 via a signalcable 124. The implant housing 100 may be utilized to house a number ofcomponents of the implantable hearing instrument. For instance, theimplant housing 100 may house an energy storage device and a signalprocessor. Various additional processing logic and/or circuitrycomponents may also be included in the implant housing 100 as a matterof design choice. In the present arrangement, the signal processorwithin the implant housing 100 is electrically interconnected to atransducer 108. In the present embodiment, the transducer 108 isconnected to the implant housing via first and second signal cables 106,107. These signal cables 106, 107 are connected by a detachableconnector 80.

As illustrated in FIG. 2, the transducer 108 is supportably connected toa positioning system 110, which in turn, is connected to a bone anchor116 mounted within the patient's mastoid process (e.g., via a holedrilled through the skull). The transducer 108 includes a connectionapparatus 112 for connecting the transducer 108 to the ossicles 120 ofthe patient. In a connected state, the connection apparatus 112 providesa communication path for acoustic stimulation of the ossicles 120, e.g.,through transmission of vibrations to the incus 122.

Referring to FIGS. 1 and 3, it is noted that microphone assembly 130 inthe present embodiment is a pendant microphone that is connected to theimplant housing via a signal cable 124. Use of such a pendant microphoneallows the microphone assembly 130 to be spaced from the implant housing100 such that it need not be mounted to the skull of a patient. Suchspacing may facilitate vibration attenuation as well as reduce thenumber of components that need to be mounted in/on the limited space onthe skull near the patient's ear (e.g., mounted near the mastoidprocess). However, it will be appreciated that in other embodiments, themicrophone may be mounted or integrally formed on or within the implanthousing 100.

The microphone assembly 130 includes a housing 134 and diaphragm 132that is positioned to receive ambient acoustic signals through overlyingtissue. During normal operation, acoustic signals are receivedsubcutaneously at the diaphragm 132 of the microphone assembly 130.Internal components of the microphone assembly 130 generate an outputsignal that is indicative of the received acoustic signals. The outputsignal is provided to the implant housing 100 via the signal cable 124.Upon receipt of the output signal, a signal processor within the implanthousing 100 processes the signals to provide a processed audio drivesignal, via the connected signal cable 106 and 107, to the transducer108. As will be appreciated, the signal processor may utilize digitalprocessing techniques to provide frequency shaping, amplification,compression, noise reduction and/or other signal conditioning, includingconditioning based on patient-specific fitting parameters. The audiodrive signal causes the transducer 108 to transmit vibrations atacoustic frequencies to the connection apparatus 112 to effect thedesired sound sensation via mechanical stimulation of the incus 122 ofthe patient. Similar processes may be utilized for cochlear stimulationdevices.

An external charger (not shown) may be utilized to transcutaneouslyre-charge the energy storage device within the implant housing 100. Suchan external charger may include a power source and a transmitter that isoperative to transcutaneously transmit, for example, RF signals to theimplanted receiver 118. In this regard, the implanted receiver 118 mayalso include, for example, rectifying circuitry to convert a receivedsignal into an electrical signal for use in charging the energy storagedevice. The external transmitter and implanted receiver 118 may eachcomprise coils for inductive coupling of signals there between. Inaddition to being inductively coupled with the implanted receiver 118for charging purposes, such an external charger may also provide programinstructions to the processor(s) of the implantable hearing instrument.

Combined Signals

FIG. 4 illustrates how pressures resulting from ambient acoustic soundsand tissue-borne acceleration are combined at an implanted microphonediaphragm 132 of an implantable microphone assembly 130. As shown, theimplanted microphone diaphragm 132 is exposed to pressure in overlyingtissue 142 that is generated externally to the patient as represented byambient sound source 40. This ambient acoustic signal (e.g., sound) fromthe sound source 40 passes through and is filtered by the tissue 142overlying the microphone diaphragm 132. The deflection of the microphonediaphragm 132 by the pressure associated with the ambient sound resultsin a desired signal component, or, microphone acoustic response. Thismicrophone acoustic response is also mixed with the pressure generatedby vibrations in the overlying tissue 142 caused by one or moreacceleration sources 50. The pressure from the acceleration source 50 islikewise filtered by the tissue 142 overlying the microphone diaphragm132 and results in an undesired signal component or microphone vibrationresponse.

The acceleration source 50 may comprise any source of tissue-bornevibrations and may include biological sources and mechanical sources.Such biological sources may include, without limitation, chewing andspeaking. One example of a mechanical source includes feedback signalsfrom the transducer 108, which in the normal course of its operation mayvibrate surrounding tissue. Such vibration may subsequently be conductedto the location of the microphone diaphragm 132.

The net effect is that the pressure variations associated with theacoustic source 40 and acceleration source 50 are summed by the normalaction of the microphone diaphragm 132. That is, pressure associatedwith each of the ambient acoustic source and acceleration source, whicharrive at the microphone diaphragm 132 through the overlying tissue 142,deflect the diaphragm 132 and generate a combined microphone response oroutput signal. That is, such an output signal is a combination of thepressure associated with the two sources 40,50.

As the microphone diaphragm 132 detects the combination of the pressurefluctuations as a single varying pressure, it is desirable for theimplanted microphone to compensate for undesired signal components(e.g., the microphone vibration response) in order to detect desiredsignal components (e.g., the microphone acoustic response) withsufficient sensitivity. Stated otherwise, it is desirable for themicrophone assembly 130 to separate ambient acoustic signals fromtissue-borne vibration-induced signals.

A number of previous systems have attempted to separately measure themotion acting on the implant housing and subsequently subtract thismotion (e.g., acceleration acting on the implant housing) from thecombined microphone response. In such systems, one element of themicrophone assembly 130 is designed to be preferentially sensitive toacceleration-induced vibration and preferentially insensitive toacoustic stimulation. Such microphones typically include a motion sensorthat is disposed within the microphone housing such that it is primarilysensitive to acceleration (i.e., non-ambient vibration acting on thehousing) while being substantially insensitive to ambient acousticsignals acting on the microphone diaphragm. In this regard, an outputresponse of the motion sensor may be removed from a combined outputresponse of the microphone diaphragm 132. That is, the output of themotion sensor may be used to estimate and cancel/subtract signalsoriginating from an acceleration source from the combined microphoneresponse. The remaining signal, which is more representative of theambient acoustic signal, is used by the implanted signal processor 104to generate a drive signal for use by a transducer to stimulate acomponent of a patient's auditory system.

Previous systems that have separately measured the responses of amicrophone diaphragm and a motion sensor include co-assigned U.S. Pat.Nos. 7,214,179; 7,522,738; 7,556,597 and 7,775,964, the entire contentsof each of which is incorporated herein by reference. These patentsprovide various systems where an output of a motion sensor is measured,scaled, and subtracted from a combined output response of a microphoneelement. That is, the microphone element generates an electrical outputsignal from which an electrical motion sensor output signal issubtracted. The necessary scaling and subtraction of the motion sensorsignal can result in the addition of electrical noise to the system,which limits the achievable gain of the hearing system. Accordingly, itwould be desirable to reduce the magnitude of the motion/accelerationresponse in the microphone output to reduce subsequent processingrequirements and/or allow for improving overall gain of the hearingsystem.

Optical Measurement

Aspects of the presented inventions are based in part on the recognitionthat an acoustic response of a microphone diaphragm may be at leastpartially isolated from a vibration response during measurement using anoptical measurement system that measures relative movement between amicrophone diaphragm and a moveable cancellation element such as a proofmass. Such a system may reduce or eliminate the need for subsequentprocessing to compensate for undesired signal components.

FIG. 5 shows a schematic/mathematical depiction of the basic operatingprinciple of a microphone assembly 130 that allows for isolating theacoustic response of a microphone diaphragm output by determining therelative movement between the microphone diaphragm and a movablecancellation element 150. The cancellation element is typically aseismic or proof mass that acts as a damped mass on a spring or othercompliant support.

As shown, the microphone assembly 130 can be modeled as a spring masssystem where the diaphragm 132 and a mass of overlying tissue is a firstmass M₁ having a first spring constant k₁. The diaphragm 132 may bepositioned immediately adjacent and facing to the skin of the patientsuch that a combined force F₁, including ambient acoustic inducedpressures and acceleration induced pressures, acts upon M₁. Thecancellation element 150 is substantially isolated from ambient acousticsignals (e.g., within an implant housing). The cancellation element isrepresented by M₂, has a second spring constant k₂, and is acted upon byF₂, which is the force due to acceleration and substantially free ofacoustic forces.

The response of the two systems M₁ and M₂ is governed by simpleharmonics. It can be shown mathematically that when the microphoneassembly 130 measures a frequency significantly higher that the resonantfrequencies of the systems M₁, k₁ and M₂, k₂, the difference in movementΔ (i.e., relative movement) between the systems may be determined and isindependent of spring rates and masses of the systems. Further, thedifference in movement Δ between the cancellation element 150 and thediaphragm 132 is caused by the differences in the forces applied tothese elements and specifically by the ambient acoustic forces appliedto the diaphragm. That is, the relative movement Δ represents theambient acoustic forces applied to the diaphragm 132 substantially freeof acceleration forces. In this regard, by measuring the relativemovement Δ between the systems M₁ and M₂ the acoustic signal may bedetermined free of or with a reduced response to acceleration inducedforces.

The present disclosure improves upon existing systems that separatelymeasure microphone output and motion sensor signals by employing opticalmeasuring to directly measure the relative movement between a microphonediaphragm and a cancellation element. Generally, the presented systemsand methods identify a change in a distance or path length between aninside surface of a diaphragm that hermetically covers an aperture in animplantable housing and a cancellation element that is compliantlymounted within the implantable housing.

In the case of an implanted microphone, if the diaphragm andcancellation element move in near unison in response to commonacceleration forces, differences in the distance or path length betweenthese elements (i.e., relative movement) will substantially correspondto the acoustic forces acting on the microphone diaphragm. Therefore, toaccurately measure the change in the path length and hence isolate theacoustic response, it is desirable that the diaphragm and cancellationelement react similarly to acceleration forces. That is, it may bedesirable that the magnitude of the cancellation element response toacceleration substantially match the response of the diaphragm toacceleration. Likewise, the phases should be substantially matched aswell in order to achieve enhanced acoustic isolation. It may bepreferred that such magnitude and phase matching occur in a frequencyrange of interest (e.g., an acoustic hearing range). This may requirethat the resonant frequency of each the diaphragm and cancellationelement be less than about 2000 Hz and more preferably less than about200 Hz. These resonant frequencies are typically below an acoustichearing frequency range. Further, it may be desirable that the diaphragmand cancellation element have substantially equal resonant frequenciesand/or equal damping factors. As will be appreciated, with appropriatechoice of spring rates for the diaphragm and cancellation elementsuspension, it is possible to match the response of the diaphragm andcancellation element and thereby make the change in a path lengthbetween these elements less sensitive to acceleration induced vibration(e.g., produce near unitary movement), while preserving sensitivity toacoustic pressures acting on the diaphragm. The resulting system mayhave fewer components, may be manufactured more compactly andinexpensively and offer signal-to-noise ratio improvement in comparisonto systems that measure a combined response of a microphone andsubsequently cancel a motion signal from the combined response.

FIGS. 6A and 6B illustrate one embodiment of a microphone assembly 230that allows for optically isolating an acoustic response of a microphonediaphragm. As shown, the microphone assembly 230 includes a microphonediaphragm 232, which is responsive to acoustic signals and vibrationinduced acceleration, and a compliantly supported proof mass 240, whichis primarily responsive to vibration induced acceleration. Themicrophone diaphragm 232 serves as a hermetic seal for the microphonehousing 234 and thereby, upon implant, isolates an internal chamber ofthe microphone assembly 230 from the user's body. The proof mass 240 isa motion sensitive element that is suspended compliantly within thehousing to move relatively to the housing in response to accelerationacting on the housing. The suspension can be (for example) a set ofcoil, leaf, or pneumatic springs. In the embodiment shown, the proofmass is mounted within a ring magnet 241 polarized along its axis ofsymmetry and suspended between two fixed ring magnets 242 and 244polarized so as to maintain the proof mass 240 statically between them.Magnetic suspension, as shown here, has the advantage that the springrate is small near the center of excursion, permitting relatively largemovements in response to vibration. The proof mass 240 may be guided, asshown here, by a post 246 extending through a hole in its center.

Generally, the displacement/movement of the microphone diaphragm 232 andthe proof mass 240 are designed to be substantially equal in response toacceleration forces. In contrast, displacement of the diaphragm 232 dueto acoustic forces, which do not act on the proof mass, will result inrelative movement between the diaphragm 232 and the proof mass 240 andthereby change a path length there between.

In the present embodiment, the relative movement between the microphonediaphragm 232 and the proof mass 240 is measured using an output beamfrom a laser source 250. More specifically, the output beam travels apath between the laser source 250 and a photo detector 260 where thepath includes at least one reflection off the diaphragm 232 and at leastone reflection off of the proof mass 240. In this regard, non-unitarymovement (e.g., relative movement) between the diaphragm 232 and proofmass 240 alters the path length between the laser source 250 and thephoto detector 260. This change in the path length induces frequency andamplitude shifts (e.g., Doppler shifts) in the output beam. That is, themovement of the diaphragm relative to the cancellation element, whichmay correspond primarily to ambient acoustic forces acting on thediaphragm 232, modulates the output beam. Demodulating an output signalof the photo detector provides a signal that is indicative of thevibration of the microphone diaphragm 232 in response to ambientacoustic signals (e.g., sound) with a reduced response to accelerationinduced-vibration. Stated otherwise, the output beam from the lasersource allows for more directly measuring relative movement Δ betweenthe diaphragm 232 and the proof mass 240 and generating an acousticresponse output signal that may be utilized for hearing augmentationpurposes.

The embodiment of FIGS. 6A and 6B utilizes a laser Doppler velocimeter(LDV) configuration for measurement. In this configuration, the lasersource directs a first beam at the surfaces of interest or vibratingtargets (e.g., diaphragm and proof mass), such that relative vibrationamplitude and frequency are extracted from the Doppler shift of thelaser beam frequency due to the relative motion of these vibratingsurfaces. The LDV is generally a laser interferometer that employs twoor more laser beams to measure the frequency (or phase) differencebetween a reference beam and a test beam. The test beam is directed tothe target, and scattered light from the target is collected andinterfered with the reference beam on a photodetector, typically aphotodiode. Most vibrometers also add a known frequency shift (typically30-40 MHz) to one of the beams. This frequency shift may be generated bya Bragg cell or other acousto-optic modulator. The output of an LDV isgenerally a continuous analog voltage that is directly proportional tothe target velocity component along a direction of the laser beam.

Light from the target is reflected to the photo detector where it iscombined with the reference beam. The initial frequency of the laser istypically very high (>10¹⁴ Hz), which is higher than the response of thedetector. The detector does respond, however, to the beat frequencybetween the beams. The output of the photodetector is a frequencymodulated (FM) signal, with the Bragg cell frequency as the carrierfrequency, and the Doppler shift as the modulation frequency. Thissignal can be demodulated to derive the velocity vs. time of thevibrating target. By comparing the shift in the laser beam to areference laser beam (i.e., without a Doppler shift) the vibrationprofile/signature of the surface may be determined. Thus, by reflectingthe laser beam off of both the microphone diaphragm and the proof massit is possible to directly measure the relative movement between theseelements which, as discussed above, corresponds to the acoustic forcesimpinging on the diaphragm. In this regard, the acoustic response can beisolated from a combined output using a single sensing device.

Referring again to FIGS. 6A and 6B, it is noted that in the presentembodiment, the laser source 250 (for example, a diode laser) emits abeam passing through in sequence, a first beam splitter 252, a Braggcell 254, and a second beam splitter 256. Thus, the present embodimentsplits the initial output beam into three beams (B_(r), B_(c) andB_(t)). Specifically, the first beam splitter 252 splits the initialoutput beam of the laser source into two beams having identicalcharacteristics (e.g. frequency, amplitude, etc.). One of those beams isdesignated the reference beam B_(r) and is provided to the photodiode260 free of reflection off of the vibrating surfaces 232, 240. The otheroutput of the first beam splitter travels through the second beamsplitter 256 forming two beams: a comparison beam B_(c) and test beamB_(t). The comparison beam B_(c) and test beam B_(t) are both shifted infrequency by the Bragg Cell. In the present embodiment, the referencebeam Br travels a path to the photodiode 260 via a mirror 258 and athird beam splitter 262. Upon exiting the second beam splitter 256, acomparison beam B_(c) is also transmitted to the third beam splitter 262for provision to the photodiode 260. The two beams B_(c) and B_(r) arecombined at the third beam splitter 262 and directed to the photodetector 260.

The test beam B_(t), after exiting the second beam splitter 256 travelsan optical path that reflects off of both the proof mass 240 and aninner surface of the diaphragm 232 to a mirror 258, which directs thebeam to the photodiode 260 via the third beam splitter 262. To permitsuch a travel path, the inside surface of the diaphragm and at least onesurface of the proof mass have reflective surfaces. In this regard, suchsurfaces may be polished to achieve a desired reflectance or thesesurfaces may incorporate a reflective coating.

The Bragg cell 254 introduces a frequency shift to the test beam andcomparison beam, causing the combined beams B_(c) and B_(r) to interfereat a beat frequency equal to the Bragg cell shift. Non-unitary movementof the diaphragm 232 and proof mass 240 alters the path length of thetest beam B_(t) and thereby introduce a Doppler shift to the beam. Thephoto detector 260 receives the beat frequency modulated by the Dopplershift determined by the apparent velocity, or change in path length,with time of the test beam B_(t). As noted, the photo detector generatesa frequency modulated signal that may be demodulated to yield a signalcorresponding to the apparent velocity along B_(t). This apparentvelocity is the acoustic response of the microphone diaphragm.

FIG. 7 illustrates another embodiment of a microphone assembly 330 thatallows for isolating an acoustic response of a microphone diaphragm.Similar to the embodiment of FIG. 6A the microphone assembly 330includes a microphone diaphragm 332, which is responsive to acousticsignals and vibration induced acceleration, and a compliantly supportedproof mass 340, which is primarily responsive to vibration inducedacceleration. The proof mass 340 is a motion sensitive element that ismounted within a ring magnet 341 polarized along its axis of symmetryand suspended between two fixed ring magnets 342 and 344 polarized so asto maintain the proof mass 340 statically between them. Otherarrangements may utilize other compliant supports such as coil, leaf, orpneumatic springs.

As above, the displacement/movement of the microphone diaphragm 332 andthe proof mass 340 are designed to be substantially equal in responseacceleration forces and differ in response to acoustic forces. That is,acoustic forces act primarily on the diaphragm 332 and result inrelative movement between the diaphragm 332 and the proof mass 340 andthereby change a distance or path length there between.

The relative movement between the microphone diaphragm 332 and the proofmass 340 is measured using an output beam from a laser source 350. Morespecifically, the output beam travels a path between the laser source350 and a photo detector 360 where the path includes at least onereflection off the diaphragm 332 and at least one reflection off of theproof mass 340. In this regard, non-unitary movement (e.g., relativemovement) between the diaphragm 332 and proof mass 340 alters the pathlength between the laser source 350 and the photo detector 360.

In the present embodiment, the change in the path length is measured bymonitoring the change in a diameter of a convergent laser beam asmeasured at a receiving photo detector 360. In such a convergent laserbeam, the output beam may be a conical beam that converges at a focalpoint a known length from the laser source. FIG. 8 illustrates such aconverging beam. As shown, the beam 370 emits from a laser source 350having an initial diameter D. Further, the beam 370 converges at aconstant angle α until it converges at focal point F. After passingthought the focal point F the laser beam 370 diverges. Importantly, thediameter d of the beam at any length L measured from the source 350 canbe calculated:d=D−Lα  eq. 1The ability to calculate the diameter at any location along the beamalso permits calculating a change in a path length by monitoring achange in the diameter d of the beam at a fixed location.

Referring again to FIG. 7, it is noted that the output beam 370 of thelaser source 350 reflects off of the diaphragm 332, proof mass 340 andis received by a photo detector 360. When the system is static (i.e.,diaphragm 332 and proof mass are not moving), the photo detector 360receives the beam 370 having fixed diameter d₁, and generates an outputindicative of this beam diameter. When the diaphragm and proof mass movein unison, the path length between the source 350 and photo detector 360is unchanged and the diameter of the received beam remains unchanged(e.g., d₁), though the position of the beam on the photo detector maychange. In contrast, when there is relative movement between thediaphragm 332 and proof mass 340 due to acoustic forces acting on thediaphragm 332, the path length between these elements changes. Thechange in path length changes the diameter of the beam received by thephoto detector 360 (e.g., d₂). Likewise, the output of the photodetector also changes. Accordingly, as the initial diameter D andconvergence angle α are known and fixed, knowledge of the change ofdiameter at the photo detector allows for calculating the change of thepath length as a function of time. Based on known geometricrelationships (e.g., beam reflectance angles, static path length, etc.)the displacement of the diaphragm to the proof mass (e.g., relativemovement) can be derived. Again, this displacement is indicative of theacoustic forces acting on the diaphragm having a reduced response toacceleration induced vibration in overlying tissue.

In either of the above-noted embodiments, the isolated output signal ofthe microphone may be utilized to generate a drive signal for operatinga transducer of an implantable hearing instrument. In some instances,the isolated signal may be utilized with minimal additional processing.However, in other arrangements, additional processing may be performed.Such additional processing may include subtraction/cancellation of ameasured motion signal to further reduce the motion response in theoutput signal of the microphone. In such arrangements, it will beappreciated that the motion of the proof mass may be monitored togenerate such a motion signal. Additionally or alternatively, signalsfrom other motion sensing elements may be utilized.

Those skilled in the art will appreciate variations of theabove-described embodiments that fall within the scope of the invention.For instance, it may be advantageous to mount a laser source or detectorto a cancellation surface. In such an arrangement, the source ordetector may move in response to acceleration acting on the housing.Such an arrangement may simplify beam reflectance and/or minimize thenumber of reflections between the source and detector. However, it willalso be appreciated that additional reflections between the diaphragmand cancellation surface may allow for increasing the sensitivity of thesystem. In this regard, it may be desirable to increase the number ofreflections off of each surface. As a result, the invention is notlimited to the specific examples and illustrations discussed above, butonly by the following claims and their equivalents.

The invention claimed is:
 1. A microphone, comprising: an opticalsystem, wherein the microphone is configured such that at least one of:light is reflected off of a first component configured to move inresponse to energy input originating from at least sound and the lightreflected off of the first component is reflected off of a secondcomponent configured to move in response to acceleration of themicrophone; or light is reflected off of the second component and thelight reflected off of the second component is reflected off of thefirst component.
 2. The microphone of claim 1, further comprising: aprocessor operative to: receive an output signal indicative of receivedreflected light off of the first and second component; and process saidoutput signal to determine a vibration signature of said firstcomponent.
 3. The microphone of claim 2, wherein said vibrationsignature of said first component comprises a vibration response of saidfirst component in response to acoustic forces in an overlying mediaoverlying the first component, and has a reduced response to saidacceleration relative to that which would otherwise be the case.
 4. Themicrophone of claim 2, wherein said processor is further operative to:generate a stimulation signal using said vibration signature, saidstimulation signal being operative for actuating an actuator of animplantable hearing instrument.
 5. The microphone of claim 2, whereinsaid processor is further operative to: remove a measured signalindicative of said acceleration acting on said microphone from an outputsignal of an optical detector configured to receive at least a portionof the light reflected off of the first and second component.
 6. Themicrophone of claim 1, further comprising: a beam splitter, wherein saidoptical system is configured such that an output beam of a light sourcepasses through said beam splitter to generate a first output beam and asecond output beam, wherein said first output beam travels a path thatincludes the reflections off of the first and second components and saidsecond output beam travels a second optical path bypassing interactionwith the first and second components.
 7. The microphone of claim 6,further comprising: a frequency shift element adapted to provide afrequency shift to light passing there through, wherein a travel path ofsaid first output beam passes through said frequency shift element. 8.The microphone of claim 7, further comprising: a second beam splitter,wherein said first output beam passes through said second beam splitterafter passing through said frequency shift element, wherein said beamsplitter generates a third output beam.
 9. The microphone of claim 8,further comprising an optical detector, wherein the optical system isconfigured such that said third output beam is received by said opticaldetector that also receives the light reflected off the first and secondcomponents.
 10. The microphone of claim 1, wherein: the microphone isconfigured to be implanted in a human.
 11. The microphone of claim 10,wherein said response comprises an acoustic response of a microphonediaphragm.
 12. The microphone of claim 1, further comprising an opticaldetector configured to receive at least a portion of the light reflectedoff of the first and second components and output a signal indicative ofsaid received portion, wherein said output signal of said opticaldetector comprises: an output indicative of a Doppler shift applied tosaid received portion having traveled along an optical path includingthe reflections off of the first component and the second component. 13.The microphone of claim 1, further comprising an optical detectorconfigured to receive at least a portion of the light reflected off ofthe first and second component and output a signal indicative of saidreceived portion, wherein said output signal of said optical detectorcomprises: an output indicative of a diameter of said received portion.14. The microphone of claim 1, wherein said microphone furthercomprises: a housing including an internal chamber; an apertureextending through a sidewall of said housing; and a diaphragm disposedover said aperture, wherein the diaphragm corresponds to the firstcomponent, and at least a portion of the optical system is located inthe internal chamber of the housing.
 15. The microphone of claim 14,wherein said first and second components each have a resonant frequencyof less than about 2000 Hz.
 16. The microphone of claim 1, wherein: themicrophone includes a housing having an internal chamber; the firstcomponent is a flexible housing portion configured to move in responseto pressure variations present in a media overlying the flexible housingportion; the second component is a cancellation element disposed withinthe internal chamber of the housing; at least a portion of the secondcomponent moves relative to the first portion in response toacceleration acting on said housing; and the microphone further includesa light source configured to generate an output beam and an opticaldetector configured to receive at least a portion of said output beamvia a first optical path and to generate an output signal indicative ofsaid portion of said output beam as received by said optical detector,wherein said first optical path between said light source and saidoptical detector includes the reflections off of the first and secondcomponents.
 17. The microphone of claim 1, wherein said first and secondcomponents have substantially equal resonant frequencies.
 18. Themicrophone of claim 1, wherein said second component comprises acompliantly suspended proof mass.
 19. The microphone of claim 18,wherein said proof mass is magnetically suspended.
 20. A hearingprosthesis, comprising: the microphone of claim 1, wherein themicrophone is configured to output a signal indicative of a change in aproperty of the light reflected off of the first and second componentsdue to relative movement between the first and second components; and anoutput transducer adapted to stimulate an auditory component inaccordance with said outputted signal.
 21. The prosthesis of claim 20,wherein said output transducer comprises at least one of: a vibratoryactuator; a cochlear electrode; and a bone anchor.
 22. A microphone,comprising: an optical system including a light beam; and a microphonevibrational component configured to vibrate in response to energy fromsound waves, wherein the optical system is configured such that thelight beam reflects off of a reflective surface that moves in at leastsubstantial unison with the microphone vibrational component, whereinthe microphone is configured such that movement of the microphonevibrational component modulates the light beam reflecting off of thereflective surface, and the microphone is configured to output a signalindicative of the vibration of the reflective surface, and thus themicrophone vibrational component, based on the modulated light beam. 23.The microphone of claim 22, wherein: the microphone includes a componenthaving a mass substantially greater than the vibrational component thatmoves relative to the vibrational component; the microphone isconfigured such that movement of the mass modulates the light beamreflecting off of a reflective surface that moves due to movement of themass; and the microphone is configured to output a signal indicative ofthe vibration of the reflective surface, and thus the microphonevibrational component, based on the modulated light beam modulated dueto the vibration of the reflective surface that vibrates due to thevibration of the microphone vibrational component and due to themovement of the reflective surface that moves due to movement of themass.
 24. The microphone of claim 22, wherein: the microphone isconfigured to generate the outputted signal indicative of the vibrationof the reflective surface, and thus the microphone vibrationalcomponent, without interferometry.
 25. The microphone of claim 22,wherein: the microphone is configured to generate the outputted signalindicative of the vibration of the reflective surface, and thus themicrophone vibrational component, based on a Doppler shift of the beam.26. The microphone of claim 22, wherein: the microphone includes acomponent that moves relative to the vibrational component and iseffectively insulated from the energy from sound waves; and the opticalsystem is configured such that the light beam reflects off of areflective surface that moves in at least substantial unison with thecomponent.
 27. A hearing prosthesis, comprising: the microphone of claim22; and an output transducer adapted to stimulate an auditory componentin accordance with said signal indicative of the vibration of thereflective surface.